Fourier transform infrared (“FTIR”) spectrometers are well known in the art. Michelson interferometers function by splitting a beam of electromagnetic radiation into two separate beams via a beam splitter. Each beam travels along its own path, e.g., a reference path of fixed length and a measurement path of variable length. A reflecting element, such as a retroreflector, is placed in the path of each beam and returns both beams to the beam splitter. The beams are there recombined into a single exit beam. The variable path length causes the combined exit beam to be amplitude modulated due to interference between the fixed and variable length beams. By analyzing the exit beam, the spectrum or intensity of the input radiation may, after suitable calibration, be derived as a function of frequency.
When the above interferometer is employed in an FTIR spectrometer, the exit beam is focused upon a detector. If a sample is placed such that the modulated beam passes through it prior to impinging upon the detector, the analysis performed can determine the absorption spectrum of the sample. The sample may also be placed otherwise in the arrangement to obtain other characteristics.
Where the path length through the interferometer is varied by moving a retroreflecting element along the axis of the beam, the maximum resolution attainable with the instrument is proportional to the maximum path difference that can be produced. Because Michelson interferometers rely upon the interference from recombination of the two beams, a quality factor of such a device is the degree to which the optical elements remain aligned during path-length variation. A moving mirror is an important part of any Michelson interferometer, and the smoothness of its motion is an important part of maintaining the accuracy and stability of the spectrometer. Thus, translational displacement of the mirror must be extremely accurate. That is, the mirror must in most cases remain aligned to within a small fraction of the wavelength of incident light, over several centimeters of translation. Any deviation from pure translation may cause slight tilting of a plane mirror, leading to distortion in the detected beam. Substitution of cube-corner and cats-eye retroreflectors for plane mirrors may essentially eliminate such tilting distortion problems; but with certain inherent drawbacks.
Precision bearings may be used to maintain alignment. In addition, monitoring and controlling alignment with analysis of feedback and subsequent repositioning has been utilized to maintain mirror alignment. Typically, systems relying on either such solution have been difficult to design, relatively large, expensive and present maintenance issues.
Other efforts have been made to develop interferometers that do not require precision bearings or control systems. See, for example, U.S. Pat. No. 4,915,502 to Brierley, issued on Apr. 10, 1990, and U.S. Pat. No. 4,383,762 to Burkert, issued on May 17, 1983. However, such devices include structural deficiencies that do not provide pure, smooth translational motion, preferably with a broad arc, and substantially error-free motion regardless of orientation thereof. For example, the rigid pendulum design of the Burkert device is inherently unbalanced (which leads to various problems, such as, but not limited to, the velocity of the pendulum not being compensated, the pivot point becomes stressed, etc.), and would not operate in any orientation. While the Brierley device would also not operate in any orientation given the design thereof, further deficiencies of the complex design of the Brierley device requires an inordinate number of components, and unnecessary movement of those components, that add to economic cost and inefficiencies relating to manufacturing and maintaining same. Additionally, such devices require complex re-calibration when switching those devices from one system to another, and such complex re-calibration can include an inordinate number of steps to perform same.
So-called “porch swing” mounting arrangements are also known in the art. However, the extremely high machining tolerances required of such an arrangement and related issue of assembling same, result in high costs of both manufacture and maintenance. In addition, such pure translation flexure mounts are not typically useful for the relatively large displacements necessary for high resolution applications. The need for greater displacement can be achieved, but primarily through great cost of highly engineered precision instrumentation.
Over and above the issues raised above, the mirror-supporting structure must be isolated to the greatest possible degree from extraneous forces which would tend to produce distortions of the structure. Such forces, jerkiness, velocity variations and resultant distortions introduce inaccuracies into the optical measurements and into the motion of the optics, such as a retroreflector. The forces may arise from vibrational effects from the environment and can be rotational or translational in nature. A similarly pervasive issue concerns thermal and mechanical forces. Needless to say, considerations of weight, size, facility of use, efficiency, manufacturing cost and feasibility are also of primary importance.
Examples of external stresses that can affect the optical flatness of a reflective panel, an optical filter and/or the perpendicularity of reflective surfaces of abutting reflective panels of a hollow retroreflector, are thermal expansion or contraction of the substrate material from which the panels are made, deflection caused by curing of the adhesives used to join elements together and/or to join the retroreflector to its mount and/or the mass of the panels themselves. Accordingly, it would be desirable to assemble together the elements of a hollow retroreflector or of an optical filter, and/or to assemble a reflective panel to a device, such as a mount, quasi-translator, actuator moving mechanism, etc., in such a manner as to reduce these stresses. It would also be desirable that the manner of mounting an optical filter, reflective panel(s) and/or a retroreflector to its device not add to these stresses, but nevertheless, securely retain the optical filter, reflective panel(s) and/or retroreflector on the device. Some examples of hollow retroreflector mounts which have proven successful in maintaining the reflective surfaces in their particular orientations are found in U.S. Pat. No. 3,977,765, to Morton S. Lipkins, U.S. Pat. No. 5,122,901, to Zvi Bleier, and U.S. Pat. No. 5,335,111, also to Bleier.
There are needs to provide small front-to-back distance for compactness and to have the actuator (also referred to as the “moving mirror mechanism” or “quasi-translator”) that moves the optical device, e.g., mirror, retroreflector, etc., work in any orientation. Indeed, providing such flexibility in a compact environment is preferable.
Additionally, a problem occurs when attempting to process and control one or more light/radiation source signals of such optical assemblies. For example, in one or more embodiments, it is important that a processor produce a three (3) microsecond pulse from both zero crossings of the signal. It is not simple to do this. One way to do this involves processing a direct current (“DC”) signal with comparators with adjustable thresholds. However, the problem with this method is sensitivity to pedestal shifts. Another way to do this involves an alternating current (“AC”) signal and comparators set to zero. Unfortunately, the problem with this method is random injection of DC during turn around. Such arrangements can result in zero velocity occurrences, which can result in various problems, such as, but not limited to, instability at thresholds, manifestations of incorrect/extra sample pulses that negatively affect the motion control and data collection of the optical assemblies, etc.
Accordingly, it would be desirable to provide a quasi-translator/actuator/mirror moving mechanism for use in at least one optical assembly to achieve error-free, smooth and pure translation over a sufficiently large displacement at a reasonable cost of manufacture and maintenance. It is also desirable that: (i) the quasi-translator and/or the optical assembly be isolated from extraneous forces tending to produce optical distortions, which affect the optical movement; (ii) the quasi-translator and/or the optical assembly be compact; and (iii) the quasi-translator and/or the optical assembly works in any orientation. It would be further desirable to provide a motion control system for such quasi-translators and/or optical assemblies, to avoid the aforementioned problems, including, but not limited to, zero velocity occurrences.